The packaging industry has an interest in producing packaging materials, in particular plastic packaging materials, which may be substituted for older and more conventional materials such as metal or glass. Packaging made from metal or glass materials are virtually impermeable to moisture and humidity; packaging materials made from plastic materials are virtually always susceptible to gas and vapor permeability problems. Plastic packaging materials are oxygen permeable, and if the product carried by the packaging is oxygen sensitive, changes in taste, odor, color, texture and potency may occur within the package to degrade the quality of the product. These effects create a substantially reduced shelf life for packaged materials, in particular packaged food products, and there is therefore a need to properly evaluate such packaging materials to determine the permeability characteristics of the materials, so that the reasonable shelf life of the packaging may be predicted. Stated another way, accurately calibrated permeability tests enable the dvelopment and evaluation of various packaging materials, in order that materials displaying the best shelf life characteristics may be chosen for packaging.
It is known that the oxygen permeability characteristics of certain plastic materials is greatly affected by relative humidity conditions. It is also known that relative humidity is greatly affected by temperature and pressure variations in the environment. For example, a 1.degree. C. change in room temperature can result in a 5 percent change in relative humidity. Certain materials, such as glassine, cellophane, ethylene vinyl alcohol (EVOH), and certain forms of nylon exhibit oxygen permeability characteristics which are strongly affected by the presence of water. EVOH, in particular, is an excellent oxygen barrier under "dry" conditions, but becomes a relatively poor oxygen barrier under "wet" conditions; furthermore, the oxygen transmission rate of this material is not linear over a range of relative humidity. For example, the oxygen transmission rate through 0.6 mil. EVOH is practically zero at a 50 percent relative humidity or below, is about 20 cubic centimeters per square meter per day (cc/m.sup.2 /d) at 90 percent relative humidity, and is in excess of 125 cc/m.sup.2 /d at about 98 percent relative humidity. Therefore, in order to properly evaluate the oxygen transmission rate through materials such as EVOH, it becomes important to be able to quantify relative humidity in terms more definitive than "dry" or "wet". In particular, it is necessary to effectively measure oxygen permeability of barrier materials under very high, precisely defined relative humidity conditions, preferably in the range of 70-95 percent relative humidity (RH).
One method which has been proposed as an approach for measuring oxygen transmission rate through moisture sensitive barrier films is described as the "sandwich method." This method is described by R. C. Wood, in the Journal of Testing and Evaluation, JTEVA, Volume 12, No. 3, May 1984 (pages 149-151). The method requires the assembly of a thin, multi-layer structure in which the test specimen is enclosed between moist absorbent tissues and cover sheets of an oxygen-transparent plastic material. This "sandwich" is clamped between the two halves of a gas transmission cell which measures oxygen permeability through the test specimen. The absorbent tissues are saturated with various solutions, and an assumption is made concerning the relative humidity environment which this arrangement provides. The test gases used in conjunction with the method are dry, and the method has been used with some limited success with certain types of test specimens.
For any system to have practical utility in the field of measuring gas transmission rates through barriers under variable temperature and humidity conditions, there are a number of stringent and competing objectives to be met. First, the system must be capable of operating under a wide range of test condition temperatures; second, the system must be capable of operating under a wide range of vapor pressure test conditions; third, the system must be capable of conducting tests under any combinations of temperature and vapor pressure within the test ranges set forth. In addition, the system must be operable under normal laboratory temperature and relative humidity conditions, and it must be producible at a cost which can be justified by the test results sought. Under all of the foregoing parameters, the system must accurately measure the permeability of a test gas through a test membrane, and prior art systems have been unable to accomplish this without sacrificing one or more of these parameters.
Underlying all of the requirements for the design of such a system is the base requirement that the lowest temperature in the system must be greater than the saturated vapor pressure temperature of the vapor being generated by the system. This parameter must be complied with to assure that there will be no condensation in the system, for condensation anywhere within the system will render the system useless. The ability to satisfy this parameter, as well as the other requirements for the system, is dictated by how the temperature is controlled in the system; in particular, by controlling how and where the lowest temperature occurs in the system. In principle it is desirable to have all parts of the system at a single temperature, wherein the entire system temperature may be set to, the desired test temperature for purposes of conducting the necessary tests. The remaining test conditions could then be satisfied by merely mixing the saturated vapor with the carrier gas to the desired level, usually measured in parts per million, and delivering this mixture to the test cell for permeability measurements. Unfortunately, the system having this characteristic would undoubtably violate the cost parameter restriction, because the state of the art in instrument design prohibits the temperature control of all critical parts of the system under reasonable cost constraints.
All known prior art systems have been forced to sacrifice one or more of the optimum design objectives in order to produce a system which has at least some utility in this field. For example, a Japanese company, Hisco, has developed a system for testing oxygen permeability, designated Model RH-1, and a system for testing carbon dioxide permeability, designated RH-2, by developing a bubbler device which is temperature controlled by a water bath. The output of this bubbler device is connected to a gas testing apparatus manufactured by the assignee of the present invention, under the trade designation OX-TRAN, and a humidity sensor is connected to the output of the gas test cell in the OX-TRAN device. The system operates under the assumption that the humidity measured at the output of the test cell is equal to the humidity within the test cell, although no control is maintained over temperature.
Another system has been developed by a U.S. company, 'Atory, Inc., under the model designation A3 GHU, which utilizes a "two flow method." This system requires the mixing of a gas with no vapor content with a gas that is saturated by vapor to a desired mixing ratio. This mixed gas is delivered to a test cell for measuring permeability through a membrane. Unfortunately, there is no control of temperature in the system, and the system is therefore susceptible to condensation, although a relative humidity sensor is placed at the output of the humidity generating portion of the system, and another relative humidity sensor is placed in a return line to the humidity portion of the system. The operating theory is apparently that if the two sensors measure the same relative humidity, it can be presumed that the relative humidity in the test cell is known. However, without control over temperature this assumption is invalid under most operating conditions.
Whereas it is customary to refer to a gas below its critical temperature as a vapor, in the context of the present specirication a vapor is defined as being in equilibrium with its liquid at a predetermined pressure and temperature. For purposes of this invention the pressure is presumed to be ambient atmospheric pressure. The preferred embodiment of the present invention is intended for use with water vapor, but the general teachings of the invention are equally adaptable for use with other vapors. For example, there are many materials which would find utility in conjunction with the present invention, such as ethyl acetate, Limonene, 2-Butanone (MEK), vanillin, xylene, etc.
Relative humidity is one of the most difficult physical parameters to measure in the real world, particularly high percentage relative humidities, because it is greatly affected by pressure and temperature. Under test conditions, it is even more difficult to control, for an RH measurement taken at one position in a test instrument may be significantly different than the RH in a test chamber even several inches away from the measurement position. When gas conduits and tubes are used to convey humid gases, a temperature change of one or two degrees may be sufficient to saturate the gas to 100% RH, and the accompanying buildup of water in the measuring instrument may completely destroy the test conditions being observed. As an example of this interdependency, the American Society for Testing and Materials (ASTM) standard E-104-85 states that measurement of relative humidity to an accuracy of plus or minus 0.5 percent RH requires a temperature stability of plus or minus 0.1.degree. C. in a closed measurement chamber. Very few laboratories are temperature controlled to this extent, and therefore measurements of the type contemplated herein are required to be made in a system wherein such temperature controls may be achieved.
To a lesser extent, variations in atmospheric pressure affect relative humidity, and it has been found that a good testing procedure requires a control over ambient pressure to within about plus or minus 1 percent of atmospheric pressure. Therefore, a system for testing oxygen permeability should create a pressure drop of no greater than this value.